Coated abrasive articles, considered the premier tools for abrading and finishing plastics, wood and wood-like materials, unfortunately often suffer from the generation of static electricity during their use. The static electricity is generated by the constant interaction of the coated abrasive belt or disc with the workpiece and the back support for the belt or disc. This static charge is typically on the order of 50 to 100 kilovolts.
Static electricity is responsible for numerous problems. A sudden discharge of the accumulated static charge can cause serious injury to an operator in the form of an electrical shock or it can cause the ignition of dust particles, which poses a serious threat of fire or explosion. The static charge also causes the sawdust to cling to various surfaces, including that of the coated abrasive and electrically the non-conductive wood workpiece, thereby making it difficult to remove by use of a conventional exhaust system. Associated with this accumulation of sawdust on the coated abrasive and the wood workpiece is the further problem of "loading" of the coated abrasive (i.e., filling of the spaces between the abrasive grains with swarf). Such loading dramatically reduces the cutting ability of the abrasive grains and often results in burning the surface of the workpiece.
If the static electrical charge is reduced or eliminated, the coated abrasive article can have a significantly longer useful life, produce a finer surface finish on the workpiece and eliminate or reduce the potential for the above-mentioned hazards.
Many attempts, with varying degrees of success, have been made to solve the static electricity problem. One common approach has been to incorporate a conductive or antistatic material into the coated abrasive construction to eliminate the accumulation of electrical charge. In this regard, U.S. Pat. No. 3,163,968 (Nafus) discloses a coated abrasive article having a coating comprising graphite in a binder on the surface opposite the abrasive material. U.S. Pat. No. 3,942,959 (Markoo et al.) discloses a coated abrasive construction having a conductive resin layer sandwiched between two nonconductive resin layers to prevent the accumulation of electrostatic charge during grinding. The resin layer is made conductive by incorporating into the resin a conductive filler which may be a metal alloy, metal pigment, metal salt or metal complex. U.S. Pat. No. 3,992,178 (Markoo et al.) discloses a coated abrasive article having an outer layer comprised of graphite particles in a bonding resin which reduces the electrostatic charges generated during grinding. Japanese Unexamined Patent Publication No. 58-171264, published Oct. 7, 1983, discloses a coated abrasive article having an abrasive layer made conductive by including therein, carbon black particles having an average particle size of from 20 to 50 nanometers.
Additionally, Minnesota Mining & Manufacturing Company, the assignee of the present application, has since approximately 1975 marketed coated abrasive products under the trade designations Tri-M-ite Resin Bond Cloth Type TL and Three-M-ite Resin Bond Cloth Type TW, which contain 2% by weight carbon black and 5% by weight graphite in the adhesive size coat. The addition of the combination of carbon black and graphite to the size coat having been discovered to provide some reduction in the generation of static electricity. However, the reduction in the generation of static electricity was insufficient to prevent the sawdust from clinging to the coated abrasive article or to eliminate the risk of electrical shock. Thus there is still considerable room for improvement in reducing the generation of static electricity.